Balanced-phase-converter system.



R. R. LAWRENCE. BALANCED PHASE CONVERTER SYSTEM.

APPLICATION HLEQ JULY 24.1916- Patented Dec. 11; 1917.

UNITED STATES PATENT onrron.

RALPH R. LAWRENCE, 0F DORCHESTER, MASSACHUSETTS, ASSIGNOR T0 WESTING-HOUSE ELECTRIG & MANUFACTURING COMPANY, A CORPORATION OF PENN- SYLVANIA.

BALANCED-PHASE-CONVERTER SYSTEM.

Specification 01' Letters Patent.

Patented Dec. 11, 191%.

Application filed July 24, 1916. Serial No. 110,815.

a citizen of the United States, and resident of Dorchester, in thecounty of Suffolk and State of Massachusetts, have invented new anduseful Improvements in Balanced- Phase-Converter Systems, of which thefollowing is a specification. I

This invention relates to a system of converting current of one phaseinto current of a different phase, and more partlcularly the inventionrelates to means for maintaining proper phase relations and voltageconditions, throughout a wide range in variation of load, in a polyphasecircuit connected with a phase converter. In the preferred form theinvention comprises a single-phase circuit, a three-phase circuit, arotary phase converter connected therebetween, and reactance coils in atleast two of the branches of the three-phase circuit 1nductively linkedtogether in such a way as to maintain proper phase displacements andvoltages in the three'phase circuit throughout variations in load.However, it is not to be understood that my invention is limited tosingle-phase and three-phase systems, but it is to be understood thatthe invention is applicable to systems for converting current of anyparticular phase into current of any other desired phase.

It has been found desirable, for many purposes, to generate current ofone phase and to convert the current so generated, either at thegenerating station or at a distant point, into currentof a difierentphase, and sundry systems comprising special transformer arrangements orrotary phase converters have been designed to attain this end. One ofthe most satisfactory systems for the purpose is that illustrated anddescribed in an article published by the Westinghouse Electric & Mfg.Co. of Pittsburg, Pa, in the Electric Journal, vol. XII, No. 6, June,1915, pp. 261-264. Systems of this type, however, possess the inherentdisadvantage of disturbing the phase relations and voltage conditions ofa polyphase current to such an extent that they are impractical wherenormal phase relations and voltage conditions are important factorsunless special means is provided to balance the system. One means ofcompensating for the tendency to disturb the voltage conditions isdescribed in the article above referred to, but it is type.

desirable to. compensate for undue disturbance of the phase relations aswell as for the disturbance of the voltages. Furthermore,

it is desirable to maintain proper phase relatlons and voltageconditions not only at one particular load, such as full load orthreequarter load, but throughout the entire range of loads.

The principal object of my invention, therefore, is to provide means formaintaming proper phase relations and voltage conditions in a polyphasecircuit receiving current from, or supplying current to, a phaseconverter, particularly of the rotary Other objects of the inventionwill be apparent from the following detailed description andaccompanying drawings, in which Figure 1 is a circuit diagramillustrating one embodiment of my invention; and

Figs. 2, 3 and l are vector diagrams illustra-ting the theory ofoperation of the system.

In the particular embodiment of my invention illustrated in Fig. l, T isan ordinary single-phase transformer, RC is a rotary phase converter,and IC is a reactancecoil which may be of any approved type. Theleft-hand winding of the transformer may be connected to a single-phasecircuit and the three right hand leads L L, and ll comprise the threebranches of a threephase circuit which may be connected either to thesource of power or to the load, depending upon whether the system is tobe used to convert three-phase current .into single-phase current orvice versa. The rotary phase converter RC preferably com prises anordinary induction motor having two sets of windings w and w on thestator S and having a rotor R of either the squirrel-cage type, asindicated, or other suitable type. The stator is shown as being of theGram'me ring type merely for convenience of illustration and in practicewould of course be of suitable modern construction. One set w of theconverter windings is connected across the transformer circuit L L andthe other set w of the windings is connected between an intermediatepoint preferably the middle point, of the transformer X and the thirdphase L, of the three-phase circuit, the two sets of windings beingwound in 90-degree phase relation with each other.

A11 essential feature of the system is that na the voltage delivered tothe three-phase circuit by the phase converter be substantially 86.6 ercent. of the Volta e delivered to the t rec-phase circuit by t etransformer, when converting single-phase current to three-phasecurrent. Inasmuch as the windings w function as motor windings and thewindings w function as generator windings, thisvoltage condition can beattained in either of the following waysr'lhe windings w may beconnected directly across the transformer secondary and the windings tohave 86.6 per cent. of the turns of the windings w, as indicated in Fig.1; or, the windings a may be connected to points dis laced inwardly fromthe ends of the trans ormer secondary such distances that the voltagesupplied to the windings w is only 86.6 'per cent. of the volta e acrossthe transformer, in which event the turns of windings w are equal innumber to the turns of wind-" en converting single-phase current intothree-phase current, the no-load voltage relations'in the three-phasecircuit are as indicated in Figs. 2 and 3, wherein ac represents thesingle-phase eleetromotive force 1 delivered from the transformer andfl) represents the electromotive force generated in the windings 'w ofthe converter, the latter being 86.6 per cent. of the former. Thus, withfb at'right an les to ac, the voltages between thethree-p ase leads,represented by ab, and ca, respectively, are equal in magnitude andproperly displaced in phase with relation to each other, that is,uniformly displaced in phase 120 degrees from each' other. In theabsence of balancing means, this symmetrical no-load condition becomesunbalanced as soon as a load is applied and as the-load increases, theunbalanced condition rapidly increases, due to the change in thequadrature field, with load, and to the impedance drops in the windingsof the converter. Since one phase of the converter acts as a motor whilethe other acts as a generator, the impedance drops cannot produce thesame effect in the terminal voltage of each. phase." In

Fig. 2, for example, fb', which represents the electromotive forcegenerated in the windings w when the system is loaded, is both reducedin value and shifted in phase With relation to the electromotive forceac across the transformer circuit.

By employing reactance coil IC having windings 2', and 71, connected inleads L, and L respectively, in such relation that the magnetcmotiveforces produced by the currents in the two windings are 60 degreesapart, the phase position of the single-phase voltage, represented byac, is caused to shift along with the phase position of the voltagegenerated by the winding w and represented by fb. Thus, in Fig. '3, a'crepthroughout t eentire range of loads.

substantially a 90-degreerelatio'nship with' the voltage 7) generated bywindingsA 11 due to the impedance of the windings z, and z, the voltagea'c is reduced to approximately the proper extent to make ab, b'c and ca equal, whereby not only the proper phase relationship between thevoltages of the three-phase circuit is maintained, but the threevoltages are maintained substan tially equal in magnitude throughoutvariations in load.

A theoretical representation of the conditions in my improved system,when power is being transferred from single-phase to three-p ase, is setforth by the approximate vector diagram in Fig. 4, wherein theresistance drops are neglected in order to simplify the diagram. Thevectors ab, b0

and ca represent the no-load voltages across the three-phase line, fbrepresents the voltage. from the converter windings, w which is inapproximately 90-degree time-phase relation with the voltage acdelivered from the secondary of the transformer T, and fl) is equal to86.6 per cent. of ac. .The threephase currents under load, which areassumed to be equal and 120 degrees apart, are represented by thevectors L I and I Now the resultant magnetomotive force acting on themutual reactance IC is proportional to the sum of the currents L and Iflowing in the two windings of the reactance and, inasmuch as the fluxin the mutual reactance is approximately in time phase with theresultant magnetomotive' force, it produces a voltage in each of theleads L and L, 90 degrees behind it, as indicated by L X and I X. Thereactance voltage L X added to the voltage "0 gives the voltage f0 and LX (which is equal to I X) added to the voltage fa gives the voltage fa.The reactance voltages L X and I X each bear a 90-degree time-phaserelation with the reactance voltage N2 in the converter and if of theproper magnitude will, when load is applied, rotate the voltage ac, to anew position a 0 such that a 0 remains perpendicular to fb. The voltagefa is equal to the voltage fc and the relation, fb equals 86.6 per cent.of ac", required for balanced three-phase voltages is maintained,whereby the three-phase voltages ab, 6'0 and c a are equalized.

If the resistance drops are taken into account, the voltages willstillbe found to be balanced under balanced current loads, provided theratio of the resistance drop to the reactance drop in the mutualreactance is the same as the ratio of the total apparent resist ancedrop to the total apparent reactance drop in the converter. A diagramsimilar to Fig. 4 applies when power is transformed from three-phase tosingle-phase.

The coil 10 should be so designed. that the reactance drops across thewindings z, and i due to the mutual reactance between the windings, isjust suflicient to maintain the 90 degree voltage relation between a'cand fb. A suitable way of accurately adjusting the amount of mutualreactance of coil 10 comprises adjusting the length of the core air gapshown in Fig. 1. In the event that the proper balancing efiect is notobtained the connections to one of the windings 71,, or 71 should bereversed.

I claim:

1. A balanced phase converter system comprising a phase converter, acircuit for supplying current of one phase to the converter, a circuitfor delivering current of a different phase to the load, one of thecircuits being a three-phase circuit, and coils in two of the branchesof the three-phase circuit inductively connected together in degreerelation with each other.

2. A balanced phase converter system comprising a rotary phase converterhaving at least two sets of windings, a transformer, a single-phasecircuit connected to one side of the transformer, the other side of thetransformer being connected to one set of the converter windings, athree-phase circuit having two leads directly connected to the latterside of the transformer, the other set of converter windings beingconnected to an intermediate point of the latter side of the transformerand to the other lead of the three-phase circuit, and a mutual reactancecoil having windings in two leads of the three-phase circuit.

3. A balanced phase converter system comprising a rotary phase converterhaving at least two sets of windings, a transformer, a single-phasecircuit connected to one side of the transformer, the other side of thetrans former being connected to one set of the converter windings, athree-phase circuit having two leads directly connected to the latterside of the transformer, the other set' of converter windings beingconnected to an intermediate point of the-latter'side of the transformerand to the other lead of the three-phase circuit, and coils in two leadsof the three-phase circuit inductively connected together in GO-degreerelationship.

4:. A phase-converter system comprising a source of single-phasealternating-current, a three-phase load circuit, a phase-convertingdynamo-electric machine, two mains of said three-phase circuit beingconnected directly to said source and the third main traversing saidmachine, and coils in said two directly connected mains inductivelyconnected together in (SO-degree relation with each other.

5. A balanced phase-converter system comprising a rotary phase converterhaving at least two sets of windings, a transformer, a single-phasecircuit connected to one side of the transformer, the other side of thetransformer being connected to one set of the converter windings, athree-phase circuit having two leads directly connected to the latterside of the transformer, the other set of converter windings beingconnected to an intermediate point of the latter side of the transformerand to the other lead of the three-phase circuit, and a mutual reactancecoil having windings in the two leads of the three-phase circuit whichare directly connected to said transformer.

6. A balanced phase-converter system comprising a rotary phase converterhaving at least two sets of windings, a transformer, a single-phasecircuit'connected to one side of the transformer, the other side of thetransformer being connected to one set of the converter windings, athree-phase. circuit having two leads directly connected to the latterside of the transformer, the other set of converter windings beingconnected to an intermediate point of the latter side of the transformerand to the other lead of the three-phase circuit, and coils in the twoleads of the three-phase circuit which are directly connected to saidtransformer, said coils being inductively connected together in(SO-degree relationship.

7. The combination with an unbalanced three-phase system embodyingelectromotive forces of ma or, lntermediate, and minor values,respectively, of a transformer hav ing considerable magnetic leakage andconnected to derive energy from the phase of said major electromotiveforce and supply- 1ng said energy to the. phase of said minorelectromotlve force to such a degree as to balance said system.

Signed by me at Boston, Massachusetts, this 20th day of July, 1916.

, RALPH R. LAWRENCE.

